1. Field of the Invention
The present invention relates to an imaging optical system, imaging device and imaging system.
2. Description of the Related Art
An omnidirectional imaging device in which two imaging optical systems each including a wide-angle lens having an angle of view wider than 180 degrees and an imaging sensor for imaging an image by the wide-angle lens are combined, and an image by each of the imaging optical systems is synthesized to obtain an image in a solid angle of 4π radian is conventionally known (refer to Japanese Patent Application Publication No. 2010-271675).
Such an omnidirectional imaging device is able to obtain omnidirectional image information at one time, so it can be effectively used for a monitoring camera, car-mounted camera or the like. Recently, such an omnidirectional imaging device is required to be downsized so that it can be used as a portable omnidirectional imaging device.
For example, in the case of coverage, extremely accurate image information can be obtained by using a portable omnidirectional imaging device in a handheld state.
In order to achieve a small omnidirectional imaging device, it is necessary to downsize a wide-angle lens for use in an imaging optical system. It is also necessary for a wide-angle lens to have a reasonable performance in order to obtain a preferable image. Thus, it is difficult to reduce the number of lenses constituting the wide-angle lens.
Upon a certain amount of increase in the number of lenses constituting a wide-angle lens, the entire length of the wide-angle lens is increased. If two wide-angle lenses are combined to be opposite to each other, a non-photographable space where the maximum angle of view light beams entering in the respective wide-angle lenses do not overlap to each other is increased unless the angle of view is considerably increased.
If the angle of view is increased for downsizing a wide-angle lens, a part of an incident light beam is blocked by a substrate on which an imaging sensor is mounted, so that a part of the substrate is photographed on an image by the wide-angle lens. Not only an imaging sensor, but also a circuit element for driving the imaging sensor and a circuit element required for electric connection with an external device are mounted on the substrate. Consequently, the size of the substrate is inevitably increased larger than the size of the imaging sensor.
Japanese Patent Application Publication No. 2010-271675 does not disclose a technique relative to vignetting of an incident light beam by such a substrate.